Renal Calculations

Calculation of Renal Processes: Filtration, Secretion, and Reabsorption

Healthy subjects participate in a clinical research project designed to examine the renal handling of

various substances. The results of the project are given below.

Fill in the chart and determine how Substances W, X, Y, and Z were handled by the kidneys.

It is known that 50% of substance A traveled bound to plasma proteins. All other substances traveled free

in the circulation. Subject's urine flow rate (V) was determined to be 0.6 mL/min.

S = Substance ( i.e., W, X, PT, Y or A)

Net Reabsorption

[Urine S] [plasma S] Us x V Clearance Filtered load or Net Secretion

( mg/mL) (mg/mL) (mg/min) (mL/min) (mg/min) (mg/min)

Inulin 5.5 0.025 3.3 132 3.3 F=E=0

W 10.0 0.5 6.0 12 66 F>E, net Reabsorp 60

X 0.9 0.045 0.54 12 5.94 F>E net Reabsorp 5.4

PT 13 0.02 7.8 390 2.64 E>F, net Secretion5.16

Y 6.5 0.060 3.9 65 7.92 F>E net Reabsorp 4.02

A 7.5 0.085 4.5 52.94 5.61 F>E net Reabsorp 1.11

For A, ½ is bound to plasma protein and can’t be filtered therefore use ½ of 0.085 to calculate

filtered load. To calculate clearance, need to use the entire plasma concentration.

The clearance of Inulin = GFR therefore you will need to use 132 to calculate each of the filtered

loads of substances W, X, Y and Z.

To fill in the above chart you will need to remember the following:

Filtered load (mg/min) = GFR x [plasma S] mg/mL

Excreted load (mg/min) = Us (mg/mL) x V (mL/min)

Recall that 0.6 mL/min is the rate of urine formation (= V) and is given above.

If a substance is freely Filtered, not Secreted and not Reabsorbed then Filtered load = Excreted load,

using a substance that meets these criteria can be used to determine GFR by rearranging the equation.